Conventional HST (Hydrostatic Transmission) devices capable of continuously variable transmission use a hydraulic pump and a hydraulic motor. As such HST devices, an HST circuit structure (e.g., Patent Document 1), as shown in FIG. 14, are known.
As an HST device that uses one hydraulic pump and one hydraulic motor, an HST device (e.g., Patent Document 2), as shown in FIG. 16, has been proposed in which continuously variable transmission is retained during low speed running, while the hydraulic motor can be directly connected to a power source during high speed running.
In the HST circuit structure shown in FIG. 14, a variable capacity pump 50 driven by a drive source such as an engine (not shown) is connected to a fixed capacity motor 51 and a variable capacity motor 52 via circuits 59 and 60.
A gear 54 is fitted to the motor shaft 52a of the variable capacity motor 52. The gear 54 meshes with a gear 56 fitted to an output shaft 53 via a clutch 55. A gear 57 is fitted to the motor shaft 51a of the fixed capacity motor 51, and meshes with a gear 58 fitted to the output shaft 53. Rotation of the output shaft 53 rotates the tires or the like of a vehicle (not shown).
The HST device described in the Patent Document 2 has a circuit structure as shown in FIG. 16. An engine 79 drives a variable capacity pump 61. The variable capacity pump 61 and a hydraulic motor 63 form a closed circuit via oil conduits 62 and 64. The hydraulic motor 63 has an output shaft that causes a vehicle to run, and is connected to the variable capacity pump 61 via a shaft 65 to which a clutch mechanism 66 is fitted.
Normally, the clutch mechanism 66 is disconnected, in which position the hydraulic motor 63 is disconnected from the variable capacity pump 61. When the clutch mechanism 66 is connected, it connects the hydraulic motor 63 to the variable capacity pump 61. This makes it possible to directly transmit the drive force of the engine 79 to the output shaft of the hydraulic motor 63 via the shaft 65.
The clutch mechanism 66 is operated by the cylinder 67. Until the pressure in the rod-side chamber 67a of the cylinder 67 is higher by a predetermined degree than that in the bottom chamber 67b, the clutch mechanism 66 remains disengaged and the hydraulic motor 63 is disconnected from the variable capacity pump 61. When the pressure in the rod-side chamber 67a of the cylinder 67 is higher by a predetermined degree than that in the bottom chamber 67b, the thrust of the cylinder 67 engages the clutch mechanism 66, thereby connecting the output shaft of the hydraulic motor 63 to the variable capacity pump 61.
When the vehicle is running forward, the bottom chamber 67b of the cylinder 67 is supplied with pressure by a first control pump 68 connected to the variable capacity pump 61, and the rod-side chamber 67a of the cylinder 67 is supplied with pressure by a second control pump 70 connected to the hydraulic motor 63.
Further, a communication line 72, by which the oil conduits 62 and 64 communicate, is disposed via a communication valve 73. The communication valve 73 kept in a normal position by a spring 78 is in a disconnected position which blocks the communication line 72. When the solenoid 74 is excited, the communication valve 73 switches to the communicating position which activates the communication line 72. The solenoid 74 is connected to the switch 75 engaged with the swash plate of the variable capacity pump 61.
When the swash plate of the variable capacity pump 61 is inclined to a set angle within the forward running range of the vehicle, the switch 75 detects the inclination, thereby exciting the solenoid 74. Specifically, the switch 75 excites the solenoid 74 when the vehicle is running forward at high speed such that the set angle approaches the maximum in the forward running range of the vehicle, specifically, substantially when the flow rate of pressure oil delivered by the variable capacity pump 61 is at its maximum.
When the degree of inclination of the swash plate of the variable capacity pump 61 is included, from zero to the set angle, the number of revolutions of the hydraulic motor 63 can be controlled by the quantity of pressure oil delivered by the variable capacity pump 61. This makes it possible, by continuously variable transmission, to increase the speed of the vehicle.
At this time, the quantity of pressure oil delivered by the variable capacity pump 61 is less than the capacity of the hydraulic motor 63. This means that the variable capacity pump 61 must be rotated more than once in order for the hydraulic motor 63 to make one rotation.
This results in a difference between the number of revolutions of the first control pump 68 and of those of the second control pump 70, with the result that the first control pump 68 delivers more pressure oil than the second control pump 70 by a quantity corresponding to the difference in the number of revolutions. This prevents pressure in the rod-side chamber 67a of the cylinder 67 from becoming higher than that in the bottom chamber 67b by a predetermined pressure. This makes it possible to keep the clutch mechanism 66 disengaged.
When the swash plate of the variable capacity pump 61 is inclined to the maximum angle, the flow rate of pressure oil delivered by the variable capacity pump 61 matches that of the capacity of the hydraulic motor 63. In other words, the number of revolutions of the first control pump 68 is equal to that of the second control pump 70, so the first and second control pumps 68 and 70, each having the same capacity, deliver the same quantities of pressure oil.
The degree of the opening of a second orifice 71 located upstream of the rod-side chamber 67a of the cylinder 67 is smaller than that of the opening of a first orifice 69 located upstream of the bottom chamber 67b of the cylinder 67. Accordingly, pressure in the rod-side chamber 67a of the cylinder 67 exceeds that in the bottom chamber 67b by a predetermined pressure, thereby switching the clutch mechanism 66 to the engaged position.
This enables the hydraulic motor 63 to connect to the variable capacity pump 61. At this point, the switch 75 determines that the swash plate of the variable capacity pump 61 has been inclined to the set angle. In response to a signal from the switch 75, the solenoid 74 is excited and the communication valve 73 is switched to the communicating position.
When the swash plate of the variable capacity pump 61 is inclined to the maximum angle, the clutch mechanism 66 can be switched to the engaged position. Accordingly, the drive force of the engine 79 can be directly transmitted to the hydraulic motor 63 via the shaft 65.
Patent Document 1: Japanese Patent Application Laid-Open 2-240442
Patent Document 2: Japanese Patent Application Laid-Open